A fuel supply system of an internal combustion engine functions to supply the engine with fuel from a fuel tank. The fuel is pumped by a fuel pump out of the fuel tank via a pressure line to a fuel distributor having injection valves. The fuel distributor is located on the engine. A pressure sensor is usually mounted on the fuel distributor or at another location in the fuel supply system. With the pressure sensor, the fuel pressure in the fuel supply system is measured and transmitted to a control. The control maintains the pressure in the fuel supply system, especially in the fuel distributor, at a pregiven value. The fuel quantity, which is not needed by the engine, is usually returned to the fuel tank via a return line.
The fuel supply system can be configured as a high-pressure fuel supply system, especially a common-rail storage injection system for a direct-injecting engine wherein a fuel high-pressure store is provided as a fuel distributor. In common-rail storage injection systems, fuel is first supplied from the fuel tank to a downstream high-pressure pump via a presupply pump configured as an electric fuel pump. The high-pressure supply pump pumps the fuel at a very high pressure into the fuel high-pressure store and from there, the fuel reaches a combustion chamber of the engine via the injection valves configured as injectors. Pressure sensors are mounted in the fuel high-pressure store in order to measure the fuel pressure in the fuel high-pressure store for the control of the fuel pressure. One such fuel supply system is known, for example, from U.S Pat. No. 5,878,718.
From the state of the art, it is known to derive a defect of the fuel supply system from a control deviation of the control of the fuel pressure in the fuel supply system. A differentiated diagnosis of the fault with respect to individual components of the fuel supply system is not possible. It would, however, be desirable to be able to diagnose especially a fault of the fuel pump of the fuel supply system. A defective fuel pump can lead to the condition that the requested fuel pressure in the fuel supply system can no longer be reached and this, in turn, can lead to exhaust-gas relevant and power-relevant faults in the mixture formation at specific operating points of the engine.